The present invention relates to transmission diversity methods used in certain radio communication systems.
In these methods, one and the same radio signal is transmitted under different conditions by several antennas of the transmitting station, thereby improving the detection of the signal by the receiver, in particular by rendering it less sensitive to channel fading. This results in better spectral efficiency of the system.
The invention is more especially concerned with so-called closed-loop transmission diversity methods, in which the transmission parameters of the signal on the various antennas are determined from feedback data which are returned by the remote receiving station and depend on the quality of reception of the radio waves. In general, these feedback data are calculated from the estimated impulse responses corresponding to the propagation channels relating to the various antennas. The transmitting station then applies a phase shift to each antenna, so that the signals of the various antennas reach the receiving station approximately in phase.
Closed-loop transmission diversity offers a significant gain, provided that the feedback data are sent back and applied fairly rapidly. This assumes that the variations of the propagation channels are not too rapid.
A closed-loop transmission diversity method such as this is in particular envisaged in the third-generation cellular networks of the UMTS type (“Universal Mobile Telecommunications System”). A detailed description thereof is given in section 7 of the technical specification 3G TS 25.214, “Physical layer procedures (FDD)—Release 1999”, version 3.6.0, published in March 2001 by the 3GPP (“3rd Generation Partnership Project”).
A base station of such a network can have two antennas for one and the same cell and via which it broadcasts respectively two distinct beacon signals on common channels referred to as CPICHs (“Common Pilot Channels”). When the closed-loop transmission diversity is activated in respect of a mobile terminal communicating within the cell, this terminal detects the impulse responses h0, h1 individual to the two antennas on the basis of these a priori known beacon signals, and determines an optimal normalized vector g which maximizes the quantity gH.K.g., where K is the covariance matrix of the impulse responses, of the form
      K    =          [                                                                  h                0                H                            ·                              h                0                                                                                        h                0                H                            ·                              h                1                                                                                                        h                1                H                            ·                              h                0                                                                                        h                1                H                            ·                              h                1                                                        ]        ,            (      .      )        H  denoting the conjugate transpose. The direction of this optimal vector g is indicated to the base station in an uplink signalling channel. The base station applies corresponding weighting coefficients (phase and/or amplitude) in respect of the transmission by the two antennas of the dedicated signals destined for the mobile terminal in question.
In principle, the gain afforded by a closed-loop transmission diversity method such as this can be increased by equipping the base stations with more than two transmission antennas and by providing a corresponding number of beacon signals for the estimation of the impulse responses by the mobiles. However, the multiplying up of the beacon signals may be undesirable since it increases the general level of interference, given that these beacon signals are transmitted at relatively high power so as to be detected reliably in the entire cell.
Moreover, a cellular operator cannot extend the method in this manner to more than two antennas if the mobile terminals and the radio signalling interface are not modified accordingly. However, such a modification is generally outside the control of the operator.
An aim of the present invention is nevertheless to make it possible to implement a closed-loop transmission diversity method with N>M transmission antennas when the destination stations take only M beacon signals into account.